The last time Chris Waugh saw his younger sister they spent the weekend planning her 50th birthday.

Gathered in their mother Margaret's home in Durham over the Easter weekend in April, the only topic on the agenda was the celebrations ahead.

"She planned my 50th birthday and we went to Loch Lomond," said Waugh. "She hired a motor cruiser for the weekend. We had a lot of fizz – that's what Carole called champagne or prosecco, fizz – and I just wanted to do something for her, to reciprocate. I was looking forward to making a fuss and booking a restaurant.

"Carole loved entertaining and loved being entertained even more."

Speaking about the woman he refers to simply as "sister", Waugh, a bank manager from Penrith, said she was the one who had made the family proud.

"We are all from a working class background. Sister obviously wanted to improve herself and take the opportunities that life offered. She realised there were more opportunities in London and she came down here in her mid-20s.

"Sister was always very focused and driven."

Her life in London was something of a mystery to Waugh, who visited her two or three times a year, going to her favourite haunts, the Royal Albert Hall, good restaurants, and her favourite pubs. The siblings did not talk much about their private lives. "She had a long relationship earlier in her life for 10 years, but he died and I think she was looking for the right man again," he said.

While she planned to celebrate her birthday on 20 June with her family in the north, she was also arranging a trip with a group of girlfriends to Las Vegas.

Waugh said he knew little of these London friends. He appealed for them to come forward. "I would be very keen to talk to this Las Vegas crew. Maybe they can shed some light on this."

During the years she lived in Libya, working in the oil industry, he said his sister had been promoted from PA to executive level. She had bought her flat in London many years before and when she returned in 2008 had saved enough to live comfortably. When she disappeared she had been looking forward to returning to Libya post the fall of Gaddafi and the general elections, her brother said.

Recalling the weekend they last saw each other, he said she left on the bank holiday Monday to get the train back to London. "I waved her off saying: 'We'll have a good birthday party for you back in Cumbria.'

"Shortly after that mother got her usual telephone call to say sister had arrived safely. Then immediately after that the telephone calls stopped. It was as sudden as that."

Over the next few weeks the family's concerns grew as they repeatedly sent text messages, called her mobile phone and rang her landline with no response.

"Mother was very concerned within a couple of days because the habit was they would speak literally every other day."

They filed a cause for concern report with the Metropolitan police and an officer visited her flat in Marylebone, gained entry and reported back that there was nothing untoward.

Although somewhat reassured by the report, the following day the Waughs officially reported her missing.

Over the last 10 weeks Waugh said the Met police had kept in touch with him with developments. But last Thursday things started moving very quickly when the homicide and serious crime team took over the case following 10 days in which suspicious activity on her accounts grew.

As the hunt continued for Ms Waugh, her brother reflected on the 50th birthday he was planning, which has now come and gone.

"We sent cards and presents down to her in London well in advance of 20 June, so she would get them," he said. "I don't even know where they are now.

"Our imaginations have been running wild. I've considered several options, she could have gone on a quick holiday but that became less likely as the time went, and because she would still have telephoned mother.